Remove ai_chat_history_context feature flag

Previous chat conversation is added to chat AI requests by default.

Changelog: added
EE: true

ee/config/feature_flags/development/ai_chat_history_context.yml

----
-name: ai_chat_history_context
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/122920
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/414606
-milestone: '16.1'
-type: development
-group: group::ai-enablement
-default_enabled: true

ee/lib/ee/gitlab/gon_helper.rb

@@ -27,8 +27,6 @@ def add_gon_variables
           gon.payment_form_url                 = ::Gitlab::Routing.url_helpers.subscription_portal_payment_form_url
           gon.payment_validation_form_id       = ::Gitlab::SubscriptionPortal::PAYMENT_VALIDATION_FORM_ID
         end
-
-        push_frontend_feature_flag(:ai_chat_history_context, current_user)
       end
 
       # Exposes if a licensed feature is available.

ee/lib/gitlab/llm/chain/agents/zero_shot/executor.rb

@@ -120,8 +120,6 @@ def last_conversation
             strong_memoize_attr :last_conversation
 
             def conversation
-              return [] unless Feature.enabled?(:ai_chat_history_context, context.current_user)
-
               # include only messages with successful response and reorder
               # messages so each question is followed by its answer
               by_request = last_conversation

ee/spec/frontend/ai/components/ai_genie_chat_spec.js

@@ -16,7 +16,7 @@ describe('AiGenieChat', () => {
     data = {},
     scopedSlots = {},
     slots = {},
-    glFeatures = { aiChatHistoryContext: true },
+    glFeatures = {},
   } = {}) => {
     jest.spyOn(AiGenieLoader.methods, 'computeTransitionWidth').mockImplementation();
 

ee/spec/lib/gitlab/llm/chain/agents/zero_shot/executor_spec.rb

@@ -167,19 +167,6 @@
       agent.prompt
     end
 
-    context 'when ai_chat_history_context is disabled' do
-      before do
-        stub_feature_flags(ai_chat_history_context: false)
-      end
-
-      it 'includes an empty chat' do
-        expect(Gitlab::Llm::Chain::Agents::ZeroShot::Prompts::Anthropic)
-          .to receive(:prompt).once.with(a_hash_including(conversation: []))
-
-        agent.prompt
-      end
-    end
-
     it 'includes the prompt' do
       expect(Gitlab::Llm::Chain::Agents::ZeroShot::Prompts::Anthropic)
         .to receive(:prompt).once.with(a_hash_including(prompt_version: